Robotics is a growing, and increasingly important, field in industrial, medical, scientific, and other applications. In many cases, in which a robot arm or a tool attached thereto contacts a workpiece, the force and/or torque applied must be closely monitored. Accordingly, a force/torque sensor is an important part of many robotic systems.
One conventional type of force/torque sensor uses strain gages to measure the deformation of small beams connecting two mechanical parts—one connected to the robot arm and the other connected to a robotic tool (or a mechanical coupling to the tool). For example, a central “hub,” referred to in the art as a Tool Adapter Plate (TAP) is connected to a tool. Another body arranged annularly around, and spaced apart from, the TAP, referred to in the art as a Mounting Adapter Plate (MAP), is connected to a robotic arm. The MAP and TAP are connected to each other by a plurality of relatively thin (and hence mechanically deformable) beams, arranged radially around the TAP—in some cases resembling spokes of a wheel. Relative force or torque between objects respectively attached to the TAP and MAP attempt to move the MAP relative to the TAP, resulting in slight deformation, or bending, of at least some of the beams.
Conventionally, strain gages are affixed to all four surfaces of each beam, nominally in the center of each respective surface. The gages translate tensile and compressive strains at the beams' surfaces into electrical signals. As an example of their operation, consider forces in or parallel to the plane of the TAP and MAP—i.e., a z-direction torque (Tz, using the “right-hand rule”) or an x- or y-direction force (Fxy). These forces will attempt to bend at least some of the beams to the side. In this case, a strain gage on one side of a beam will detect a compressive strain, and a gage on the opposite side of the beam will detect a tensile strain. These gages will output strong signals, of opposite polarity. Strain gages on the top and bottom surfaces of the same beam will output very weak, if any, signals. Conversely, forces attempting to move the MAP or TAP out of their common plane (Fz, Txy) will generate strong, and opposite, outputs from the strain gages on the top and bottom beam surfaces, with little contribution from strain gages on the sides. Once calibrated, signals from all four strain gages on all beams are processed together to resolve the magnitude and direction of relative force and/or torque between the robot arm and tool (and hence the force/torque applied through the tool to a workpiece).
Instrumentation of a force/torque sensor is a significant source of cost in the product, as it requires precise and highly-skilled manual labor. Instrumentation also imposes design constraints on the sensor's mechanical design, because a significant amount of physical space is required around each of the four instrumented surfaces of each beam for viewing and hand tool access. These constraints become particularly restrictive at very small sensor sizes, and can necessitate sub-optimal sensor geometry to accommodate the installation and inspection of instrumentation. Furthermore, lengthy and complex bond wire routing is required to transfer electrical signals from strain gages on all four surfaces of each beam to a central processing circuit, which may increase the risk of device failure.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.